1. Field of the Invention
The present invention relates to measuring and control of fluid flow, particularly at low fluid flow rates. More particularly, the present invention permits such measurement and control of fluid flow with a flow sensor which is accurate at fluid flow rates as low as 4-5 milliliters per minute and as high as 100 milliliters per minute.
2. Description of the Related Art
Measurement of the flow or flow rate of a fluid in a conduit, particularly, at very low fluid flow rates, has been a problem if attempted using conventional flow sensors. At very low flow many fluid sensors do not operate properly. For example, velocity flow meters such as turbine wheel flow sensors cease to operate due to there being insufficient energy in the fluid to rotate the wheel. Differential pressure flow sensors can at times operate at low flows, but the smaller flow orifices required for low flows have been prone to obstruction if there were suspended particles in the fluid. Also, pressure drops across the orifices could be significant.
Other related specialized flow sensing techniques are disclosed in U.S. Pat. Nos. 5,542,302; 5,728,949; and 7,051,757 which disclose the use of infrared light from an optical flow measurement circuit that passes through spaced holes in the rotating turbine wheel.
Although these prior flow sensing techniques patents address some of the problems of measuring and controlling flow rate of a fluid in a conduit, so far as is known, these prior flow sensing techniques are not capable of satisfactorily measuring or controlling fluid flow rate at rates less than approximately 10 milliliters per minute.